A magnetic resonance imaging apparatus is a non-invasive medical diagnostic imaging apparatus utilizing a nuclear magnetic resonance phenomenon that a hydrogen nucleus (proton) placed in a static magnetic field resonates with an RF magnetic field at a specific frequency. Since a nuclear magnetic resonance signal varies according to various physical properties such as proton density and relaxation time, there is a possibility that metabolism of a living tissue and blood perfusion are evaluated, in addition to evaluation of lesion morphology and physical evaluation. Currently, such utilization of the nuclear magnetic resonance signal is applied to evaluation of cerebral ischemia disease and tumor, and attempts are being made to predict a treatment effect and a prognosis.
In order to determine the state of cerebral ischemia disease and tumor, and a degree of severity thereof, it is clinically important to evaluate oxygen metabolism in tissues according to oxygen imaging, such as brain oxygen extraction fraction (OEF: Oxygen Extraction Fraction) and oxygen saturation (StO2: Tissue Oxygen Saturation) in tumor. Currently used clinical evaluation of oxygen metabolism sets as a gold standard, Positron Emission Tomography (PET) examination that employs 15O-labeled gas and 18F labeled fluoromisonidazole (FMISO) agent. However, the PET examination causes problems that the time of examination is long and there is exposure to radiation. On the other hand, an MRI examination features that the examination time is short and there is no exposure to radiation, allowing reduction of impacts on a patient.
There have been suggested several methods for calculating the oxygen extraction fraction by using MRI. As one of the methods, a velocity-selective pre-pulse sequence and a spin-echo sequence are used to measure blood signals in capillaries of the living tissue at a plurality of TEs. Based on the measured signal strength at the plurality of TEs, a value of T2 of the blood signal is calculated by signal fitting. Then, by the use of the relationship between the oxygen saturation of the blood obtained separately and the T2 value of the blood signal, the oxygen extraction fraction in the living tissue is calculated (e.g., see the Patent Document 1).
As alternatives, there are methods to calculate the oxygen extraction fraction from magnetic susceptibility variations in a living body. As one of those methods, there is a method that uses quantitative susceptibility mapping (QSM) to capture a variation in magnetic susceptibility reflecting the oxygen extraction fraction. The QSM is a method of calculating local magnetic field variations caused by a susceptibility difference between tissues, from a phase distribution in an MR image, and estimating a susceptibility distribution, on the basis of a relational expression between the magnetic field and the magnetic susceptibility.
When oxygen is consumed in the living tissue, oxyhemoglobin in arterial blood changes to deoxyhemoglobin in venous blood. It is known that the magnetic susceptibility of the venous blood varies linearly with respect to the concentration of deoxyhemoglobin in the venous blood. Therefore, according to the QSM method, the oxygen extraction fraction in the living tissue can be calculated according to the variation of magnetic susceptibility of deoxyhemoglobin.
For example, one of the methods for calculating the oxygen extraction fraction according to the QSM is focusing only on the susceptibility distribution in veins calculated by the QSM method, and calculating the oxygen saturation in veins based on the magnetic susceptibility (e.g., Non Patent Document 1). In another method, caffeine is administered to an examinee, and cerebral blood flow (CBF) and the susceptibility distribution are calculated by using the ASL (Arterial Spin Labeling) and the QSM method, respectively, before and after the administration of caffeine. Then, the OEF is calculated before or after the administration of caffeine, under the condition that the metabolic rate of oxygen (CMRO2) represented by a product of CBF and OEF does not change before and after the administration of caffeine (e.g., see Non Patent Document 2).